Screw pumps usually comprise two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump housing such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which typically acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface and thereby urged through the pump as the rotors rotate.
Screw pumps are widely regarded as a reliable means for generating vacuum conditions in a multitude of processes. Consequently, they are being applied to an increasing number of industrial processes. Such applications may involve materials that have “waxy” or “fatty” properties e.g. tallow based plasticisers. In operation of the pump, these products form deposits on the surfaces of the pump. On shutdown of the pump these surfaces cool, the deposits also cool and solidify within the pump. Where such deposits are located in clearance regions between components, they can cause the pump to seize up such that restart is inhibited or even prevented.
Similar problems can be encountered in a number of semiconductor processes that use vacuum pumps, especially those in the chemical vapour deposition (CVD) category. Such processes can produce a significant amount of by-product material. This can be in the form of powder or dust, which may remain loose or become compacted, or in the form of hard solids, especially if the process gas is condensable and sublimes on lower temperature surfaces. This material can be formed in the process chamber, in the foreline between the chamber and the pump, and/or in the vacuum pump itself. If such material accumulates on the internal surfaces of the pump during its operation, this can effectively fill the vacant running clearance between the rotor and stator elements on the pump, and can also cause spikes in the current demand on the motor of the vacuum pump. If this continues unabated, then this build-up of solid material can eventually cause the motor to become overloaded, and thus cause the control system to shut down the vacuum pump. Should the pump be allowed to cool down to ambient temperature, then this accumulated material will become compressed between the rotor and stator elements. Due to the relatively large surface area of potential contact that this creates between the rotor and stator elements, such compression of by-product material can increase the frictional forces opposing rotation by an order of magnitude such that rotation is prevented on restart.
In order to release the rotors in prior art pumps, a facility is provided whereby a bar can be inserted into sockets attached to the primary shaft of the rotor through an access panel. This bar is used as a lever to try to rotate the shaft and release the mechanism such that the machine can be restarted. This levering system allows more rotational force to be applied to the internal components than could be exerted by the motor. Such force will be transmitted to the rotor vanes and the associated stresses may prove to be detrimental to the structure of the rotor. If this system fails to release the mechanism it is then necessary to disassemble the apparatus such that a liquid solvent can be poured into the pump casing to dissolve the residue to a level where the shaft can be rotated manually. This disassembly not only causes the pump to be off line for a certain length of time, but it then must be re-commissioned and re-tested to ensure the reliability of the connections to the surrounding apparatus.
Our pending international application WO2004/036047 describes how the delivery of a cleaning fluid can be activated at predetermined intervals during operation of the pump, for example using solenoid valve control. The performance of the pump is monitored by measuring at least one of the group of rotor speed, power consumption and volumetric gas flow rate. These measured parameters are subsequently used to determine the extent of accumulation of deposits on the internal working surfaces of the pump. A cleaning fluid flow rate is then calculated, this rate being that of the delivered fluid that would be sufficient to compensate for the quantity of accumulated deposits. In this way the flow rate of cleaning fluid being delivered to the rotor can be continuously adjusted to reflect the new calculated value.
It is an aim of the present invention to seek to further improve the aforementioned process.